IB/ipath: Initialize diagpkt file on device init only

Don't attempt to set up the diagpkt device in the module init code.
Instead, wait until a piece of hardware is initialized.  Fixes a
problem when loading the ib_ipath module when no InfiniPath hardware
is present: modprobe would go into the D state and stay there.

Signed-off-by: Robert Walsh <robert.walsh@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>